Types & Operations

Types

TFHE-rs includes two main types to represent encrypted data:

• FheUint: this is the homomorphic equivalent of Rust unsigned integers u8, u16, ...

• FheInt: this is the homomorphic equivalent of Rust (signed) integers i8, i16, ...

In the same manner as many programming languages, the number of bits used to represent the data must be chosen when declaring a variable. For instance:

    // let clear_a: u64 = 7;
    let mut a = FheUint64::try_encrypt(clear_a, &keys)?;

    // let clear_b: i8 = 3;
    let mut b = FheInt8::try_encrypt(clear_b, &keys)?;

    // let clear_c: u128 = 2;
    let mut c = FheUint128::try_encrypt(clear_c, &keys)?;

Operation list

The table below contains an overview of the available operations in TFHE-rs. The notation Enc (for Encypted) either refers to FheInt or FheUint, for any size between 1 and 256-bits.

More details, and further examples, are given in the following sections.

name	symbol	Enc/Enc	Enc/ Int
Neg	-	✔️	✔️
Add	+	✔️	✔️
Sub	-	✔️	✔️
Mul	*	✔️	✔️
Div	/	✔️	✔️
Rem	%	✔️	✔️
Not	!	✔️	✔️
BitAnd	&	✔️	✔️
BitOr	|	✔️	✔️
BitXor	^	✔️	✔️
Shr	>>	✔️	✔️
Shl	<<	✔️	✔️
Min	min	✔️	✔️
Max	max	✔️	✔️
Greater than	gt	✔️	✔️
Greater or equal than	ge	✔️	✔️
Lower than	lt	✔️	✔️
Lower or equal than	le	✔️	✔️
Equal	eq	✔️	✔️
Cast (into dest type)	cast_into	✔️	✖️
Cast (from src type)	cast_from	✔️	✖️
Ternary operator	if_then_else	✔️	✖️

Integer

In TFHE-rs, integers are used to encrypt all messages which are larger than 4 bits. All supported operations are listed below.

Arithmetic operations.

Homomorphic integer types support arithmetic operations.

The list of supported operations is:

name	symbol	type
Neg	-	Unary
Add	+	Binary
Sub	-	Binary
Mul	*	Binary
Div*	/	Binary
Rem*	%	Binary

For division by 0, the convention is to return modulus - 1. For instance, for FheUint8, the modulus is $2^8=256$, so a division by 0 will return an encryption of 255. For the remainder operator, the convention is to return the first input without any modification. For instance, if ct1 = FheUint8(63) and ct2 = FheUint8(0) then ct1 % ct2 will return FheUint8(63).

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt8, FheUint8};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_integers().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 15_u64;
    let clear_b = 27_u64;
    let clear_c = 43_u64;
    let clear_d = -87_i64;

    let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint8::try_encrypt(clear_b, &keys)?;
    let mut c = FheUint8::try_encrypt(clear_c, &keys)?;
    let mut d = FheInt8::try_encrypt(clear_d, &keys)?;


    a = a * &b;     // Clear equivalent computations: 15 * 27 mod 256 = 149
    b = &b + &c;    // Clear equivalent computations: 27 + 43 mod 256 = 70
    b = b - 76u8;   // Clear equivalent computations: 70 - 76 mod 256 = 250
    d = d - 13i8;   // Clear equivalent computations: -87 - 13 = 100 in [-128, 128[

    let dec_a: u8 = a.decrypt(&keys);
    let dec_b: u8 = b.decrypt(&keys);
    let dec_d: i8 = d.decrypt(&keys);

    assert_eq!(dec_a, ((clear_a * clear_b) % 256_u64) as u8);
    assert_eq!(dec_b, (((clear_b  + clear_c).wrapping_sub(76_u64)) % 256_u64) as u8);
    assert_eq!(dec_d, (clear_d - 13) as i8);

    Ok(())
}

Bitwise operations.

Homomorphic integer types support some bitwise operations.

The list of supported operations is:

name	symbol	type
Not	!	Unary
BitAnd	&	Binary
BitOr	|	Binary
BitXor	^	Binary
Shr	>>	Binary
Shl	<<	Binary
Rotate Right	rotate_right	Binary
Rotate Left	rotate_left	Binary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint8};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_integers().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 164;
    let clear_b = 212;

    let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint8::try_encrypt(clear_b, &keys)?;

    a = a ^ &b;
    b = b ^ &a;
    a = a ^ &b;

    let dec_a: u8 = a.decrypt(&keys);
    let dec_b: u8 = b.decrypt(&keys);

    // We homomorphically swapped values using bitwise operations
    assert_eq!(dec_a, clear_b);
    assert_eq!(dec_b, clear_a);

    Ok(())
}

Comparisons.

Homomorphic integers support comparison operations.

Due to some Rust limitations, it is not possible to overload the comparison symbols because of the inner definition of the operations. This is because Rust expects to have a Boolean as an output, whereas a ciphertext is returned when using homomorphic types.

You will need to use different methods instead of using symbols for the comparisons. These methods follow the same naming conventions as the two standard Rust traits:

• PartialOrd

• PartialEq

The list of supported operations is:

name	symbol	type
Equal	eq	Binary
Not Equal	ne	Binary
Greater Than	gt	Binary
Greater or Equal	ge	Binary
Lower	lt	Binary
Lower or Equal	le	Binary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt8};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_integers().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a: i8 = -121;
    let clear_b: i8 = 87;

    let mut a = FheInt8::try_encrypt(clear_a, &keys)?;
    let mut b = FheInt8::try_encrypt(clear_b, &keys)?;

    let greater = a.gt(&b);
    let greater_or_equal = a.ge(&b);
    let lower = a.lt(&b);
    let lower_or_equal = a.le(&b);
    let equal = a.eq(&b);

    let dec_gt: i8 = greater.decrypt(&keys);
    let dec_ge: i8 = greater_or_equal.decrypt(&keys);
    let dec_lt: i8 = lower.decrypt(&keys);
    let dec_le: i8 = lower_or_equal.decrypt(&keys);
    let dec_eq: i8 = equal.decrypt(&keys);

    // We homomorphically swapped values using bitwise operations
    assert_eq!(dec_gt, (clear_a > clear_b ) as i8);
    assert_eq!(dec_ge, (clear_a >= clear_b) as i8);
    assert_eq!(dec_lt, (clear_a < clear_b ) as i8);
    assert_eq!(dec_le, (clear_a <= clear_b) as i8);
    assert_eq!(dec_eq, (clear_a == clear_b) as i8);

    Ok(())
}

Min/Max.

Homomorphic integers support the min/max operations.

name	symbol	type
Min	min	Binary
Max	max	Binary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint8};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_integers().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a:u8 = 164;
    let clear_b:u8 = 212;

    let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint8::try_encrypt(clear_b, &keys)?;

    let min = a.min(&b);
    let max = a.max(&b);

    let dec_min : u8 = min.decrypt(&keys);
    let dec_max : u8 = max.decrypt(&keys);

    // We homomorphically swapped values using bitwise operations
    assert_eq!(dec_min, u8::min(clear_a, clear_b));
    assert_eq!(dec_max, u8::max(clear_a, clear_b));

    Ok(())
}

Ternary conditional operator.

The ternary conditional operator allows computing conditional instructions of the form if cond { choice_if } else { choice_else }.

name	symbol	type
Ternary operator	if_then_else	Ternary

The syntax is encrypted_condition.if_then_else(encrypted_choice_if, encrypted_choice_else). The encrypted_condition should be an encryption of 0 or 1 in order to be valid.

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt32};

fn main() -> Result<(), Box<dyn std::error::Error>> {
   // Basic configuration to use homomorphic integers
   let config = ConfigBuilder::all_disabled()
        .enable_default_integers()
        .build();

	// Key generation
	let (client_key, server_keys) = generate_keys(config);
	
	let clear_a = 32i32;
	let clear_b = -45i32;
	
	// Encrypting the input data using the (private) client_key
	// FheInt32: Encrypted equivalent to i32
	let encrypted_a = FheInt32::try_encrypt(clear_a, &client_key)?;
	let encrypted_b = FheInt32::try_encrypt(clear_b, &client_key)?;
	
	// On the server side:
	set_server_key(server_keys);
	
	// Clear equivalent computations: 32 > -45
	let encrypted_comp = &encrypted_a.gt(&encrypted_b);
	let clear_res: i32 = encrypted_comp.decrypt(&client_key);
	assert_eq!(clear_res, (clear_a > clear_b) as i32);
	
	// `encrypted_comp` contains the result of the comparison, i.e.,
	// a boolean value. This acts as a condition on which the
	// `if_then_else` function can be applied on.
	// Clear equivalent computations:
	// if 32 > -45 {result = 32} else {result = -45}
	let encrypted_res = &encrypted_comp.if_then_else(&encrypted_a, &encrypted_b);
	
	let clear_res: i32 = encrypted_res.decrypt(&client_key);
	assert_eq!(clear_res, clear_a);
	
	Ok(())
}

Casting.

Casting between integer types is possible via the cast_from associated function or the cast_into method.

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt16, FheUint8, FheUint32, FheUint16};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled()
        .enable_default_integers()
        .build();
    let (client_key, server_key) = generate_keys(config);

    // Casting requires server_key to set
    // (encryptions/decryptions do not need server_key to be set)
    set_server_key(server_key);

    {
        let clear = 12_837u16;
        let a = FheUint16::encrypt(clear, &client_key);

        // Downcasting
        let a: FheUint8 = a.cast_into();
        let da: u8 = a.decrypt(&client_key);
        assert_eq!(da, clear as u8);

        // Upcasting
        let a: FheUint32 = a.cast_into();
        let da: u32 = a.decrypt(&client_key);
        assert_eq!(da, (clear as u8) as u32);
    }

    {
        let clear = 12_837u16;
        let a = FheUint16::encrypt(clear, &client_key);

        // Upcasting
        let a = FheUint32::cast_from(a);
        let da: u32 = a.decrypt(&client_key);
        assert_eq!(da, clear as u32);

        // Downcasting
        let a = FheUint8::cast_from(a);
        let da: u8 = a.decrypt(&client_key);
        assert_eq!(da, (clear as u32) as u8);
    }

    {
        let clear = 12_837i16;
        let a = FheInt16::encrypt(clear, &client_key);

        // Casting from FheInt16 to FheUint16
        let a = FheUint16::cast_from(a);
        let da: u16 = a.decrypt(&client_key);
        assert_eq!(da, clear as u16);
    }

    Ok(())
}

ShortInt Operations

Native small homomorphic integer types (e.g., FheUint3 or FheUint4) can easily compute various operations. In general, computing over encrypted data in TFHE-rs is as easy as computing over clear data, since the same operation symbol is used. The addition between two ciphertexts is done using the symbol + between two FheUint values. Many operations can be computed between a clear value (i.e. a scalar) and a ciphertext.

In Rust, operations on native types are modular. For example, computations on u8 are carried out modulo $2^{8}$. A similar idea applies for FheUintX, where operations are done modulo $2^{X}$. For FheUint3, operations are done modulo $8 = 2^{3}$.

Arithmetic operations.

Small homomorphic integer types support all common arithmetic operations, meaning +, -, x, /, mod.

The division operation presents a subtlety: since data is encrypted, it is possible to compute a division by 0. In this case, the division is tweaked so that dividing by 0 returns the maximum possible value for the message.

The list of supported operations is:

name	symbol	type
Add	+	Binary
Sub	-	Binary
Mul	*	Binary
Div	/	Binary
Rem	%	Binary
Neg	-	Unary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 7;
    let clear_b = 3;
    let clear_c = 2;

    let mut a = FheUint3::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint3::try_encrypt(clear_b, &keys)?;
    let mut c = FheUint3::try_encrypt(clear_c, &keys)?;


    a = a * &b;  // Clear equivalent computations: 7 * 3 mod 8 = 5
    b = &b + &c; // Clear equivalent computations: 3 + 2 mod 8 = 5
    b = b - 5;   // Clear equivalent computations: 5 - 5 mod 8 = 0

    let dec_a = a.decrypt(&keys);
    let dec_b = b.decrypt(&keys);

    // We homomorphically swapped values using bitwise operations
    assert_eq!(dec_a, (clear_a * clear_b) % 8);
    assert_eq!(dec_b, ((clear_b + clear_c) - 5) % 8);

    Ok(())
}

Bitwise operations.

Small homomorphic integer types support some bitwise operations.

The list of supported operations is:

name	symbol	type
Not	!	Unary
BitAnd	&	Binary
BitOr	|	Binary
BitXor	^	Binary
Shr	>>	Binary
Shl	<<	Binary
Rotate Right	rotate_right	Binary
Rotate Left	rotate_left	Binary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 7;
    let clear_b = 3;

    let mut a = FheUint3::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint3::try_encrypt(clear_b, &keys)?;

    a = a ^ &b;
    b = b ^ &a;
    a = a ^ &b;

    let dec_a = a.decrypt(&keys);
    let dec_b = b.decrypt(&keys);

    // We homomorphically swapped values using bitwise operations
    assert_eq!(dec_a, clear_b);
    assert_eq!(dec_b, clear_a);

    Ok(())
}

Comparisons.

Small homomorphic integer types support comparison operations.

Due to some Rust limitations, it is not possible to overload the comparison symbols because of the inner definition of the operations. Rust expects to have a Boolean as an output, whereas a ciphertext is returned when using homomorphic types.

You will need to use different methods instead of using symbols for the comparisons. These methods follow the same naming conventions as the two standard Rust traits:

• PartialOrd

• PartialEq

The list of supported operations is:

name	symbol	type
Equal	eq	Binary
Not Equal	ne	Binary
Greater Than	gt	Binary
Greater or Equal	ge	Binary
Lower	lt	Binary
Lower or Equal	le	Binary

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 7;
    let clear_b = 3;

    let mut a = FheUint3::try_encrypt(clear_a, &keys)?;
    let mut b = FheUint3::try_encrypt(clear_b, &keys)?;

    assert_eq!(a.gt(&b).decrypt(&keys) != 0, true);
    assert_eq!(b.le(&a).decrypt(&keys) != 0, true);

    Ok(())
}

Univariate function evaluation.

The shortint type also supports the computation of univariate functions, which make use of TFHE's programmable bootstrapping.

A simple example of how to use these operations:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint4};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_uint4().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let pow_5 = |value: u64| {
        value.pow(5) % FheUint4::MODULUS as u64
    };

    let clear_a = 12;
    let a = FheUint4::try_encrypt(12, &keys)?;

    let c = a.map(pow_5);
    let decrypted = c.decrypt(&keys);
    assert_eq!(decrypted, pow_5(clear_a) as u8);

    Ok(())
}

Bivariate function evaluations.

Using the shortint type allows you to evaluate bivariate functions (i.e., functions that take two ciphertexts as input).

A simple code example:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint2};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::all_disabled().enable_default_uint2().build();
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);

    let clear_a = 1;
    let clear_b = 3;
    let a = FheUint2::try_encrypt(clear_a, &keys)?;
    let b = FheUint2::try_encrypt(clear_b, &keys)?;


    let c = a.bivariate_function(&b, std::cmp::max);
    let decrypted = c.decrypt(&keys);
    assert_eq!(decrypted, std::cmp::max(clear_a, clear_b) as u8);

    Ok(())
}

Boolean Operations

Native homomorphic Booleans support common Boolean operations.

The list of supported operations is:

name	symbol	type
BitAnd	&	Binary
BitOr	|	Binary
BitXor	^	Binary
Not	!	Unary